Unprecedented showdown is now on cards at party convention in July
The Republican convention will be held in July in Cleveland, and it is the national gathering at which the party officially nominates a candidate for president. In recent years, it has been a highly ceremonial formality, with candidates like Mitt Romney and John McCain having locked up the nomination months prior.
Why is this year different?
With Donald Trump's loss in Wisconsin, it is highly likely that he will arrive at the nomination without the majority of delegates needed to win outright. That would set in motion the first contested convention since 1976, when then-president Gerald Ford withstood a challenge from Ronald Reagan.
What Republican power brokers are plotting this year has no precedent in the modern primary election era. They hope to wrest the nomination from Mr Trump's grasp even if he gains a strong plurality of delegates, and put forward a more palatable alternative.
What is a contested convention?
A candidate is officially nominated when they gain a majority of delegates voting at the convention. When no candidate enters the convention with a simple majority - over 50pc - of delegates committed to them, a contested convention ensues. Most delegates are allotted based on the primary result in a particular state, and are committed to follow that result on the first ballot at the convention.
On subsequent ballots, though, most become unbound, and can shift to a candidate of their choice. Most delegates awarded to candidates who have since dropped from the race, most notably Marco Rubio, will be unbound on the first ballot. Depending on how they elect to vote, they could either provide Mr Trump the majority he seeks or prevent him from achieving it.
Should subsequent rounds fail to produce a winner, the delegates can - in theory - keep on voting indefinitely, although a nominee must be selected before the four-day convention is over.
What is the difference between an open, a contested, and a brokered convention?
They are all variations of the same thing. When no candidate has a majority going into the convention, it's called an open convention. If more than one candidate is still competing at that point, you have a contested convention.
The "brokered" aspect comes in during the fight over delegates, kind of dates from a time when the party bosses were more powerful and could actual broker the outcome. People tend to use brokered and contested interchangeably.
Could someone who hasn't run in the primaries win the nomination?
For Mr Trump to be overtaken, the delegates would have to coalesce behind one of his remaining rivals, or a "white horse" candidate whose name can be entered at the convention. John Boehner, the former Speaker of the House, endorsed Paul Ryan, the current speaker, for that role.
What could the end outcome be?
It is far from certain that Mr Trump will fall short of an outright primary election majority. And more uncertain still that the efforts to oust him at the convention will be successful. The candidate himself predicted that if the establishment plot came to fruition, his supporters would start "riots".
Larry Sabato, a longtime political analyst, agrees. "Trump has set this up so it's near chaos," he says.
"Now he's telling them, 'you can choose: it's me or it's chaos'."